Could someone help me out on this? how to activate the VTAM? Any help will be highly appreciated

Unless you have suddenly become a system programmer, there is no reason for you to be doing this -- your site support group should be doing it. Scan your system log for IST messages -- if there are some there, VTAM is active, period. And if it is active, then the message you showed means the started task name is NOT VTAM but something else. What? Only your site support group (or someone working AT YOUR SITE) could possibly tell you that. There can be ramifications to changing the USS table (what VBUILD macros use the old USS table for example -- and if you don't know what I'm referring to, you should not be doing ANYTHING to USS tables). And unless the correct updates are made to VTAMLST, then you will have to repeat your change after every IPL from now on since VTAM will not keep them for you.